If you did all that, then you've done just about everything that can be done.
You can try deleting and re-adding the accounts, but that should have happened with the restore too, unless you restored from a backup.
If you restored from a backup, you could try a clean restore and basically start over. Be sure to save important documents off the iPad first.
Also, if you have any other apps on the iPad that are supposed to work with the calendar (display, add, or delete events), like Pocket Informant can, you may want to make sure those apps have been updated to iOS 5, or just delete them and see if that helps.
And that's all I can think of at the moment.
I have not heard of this being a wide spread problem, so I doubt it's bug in iOS 5. More likely it is a glitch specific to your iPad and/or setup.